This invention relates in general to circuits for controlling fuel injectors for vehicle engines and in particular to a supplemental fuel injector control circuit for varying the duration of a fuel injector pulse length.
Fuel injection provides carefully controlled metering of fuel supplied to a vehicle engine. The careful control of fuel supply enhances engine performance and mileage while reducing harmful emissions. Referring now to the drawings, there is shown in FIG. 1 a typical known circuit for controlling a fuel injector valve. The circuit includes an Engine Control Unit (ECU) 10 having a voltage control port 12 that is connected to a first end of a fuel injector coil 14. A second end of the injector coil 14 is connected to a voltage supply V+, which for a vehicle engine, is typically a 12 volt battery. The ECU 10 includes an electronic switch for controlling the fuel injector, which in FIG. 1 is a Metal Oxide Semiconductor Field Effect Transistor (MOSFET) 16. The first end of the injector coil 14 is connected to the drain terminal of the MOSFET 16 while the MOSFET source terminal is connected to ground. The gate terminal of MOSFET 16 is connected to a control port 18 of an ECU microprocessor 19 and the MOSFET source terminal is connected through a fault detection circuit 20 to a feedback port 22 on the ECU microprocessor. When the fuel injector control MOSFET 16 is placed into a conducting state, the voltage V+ is applied across the fuel injector coil 14 and a current flows through the coil causing a magnetic field that opens the fuel injector to supply fuel to an associated engine cylinder. Likewise, when the MOSFET 16 is placed into a non-conducting state, the voltage V+ is removed from across the fuel injector coil 16, the current flowing through the coil ceases and the fuel injector closes, interrupting the supply of fuel to the associated engine cylinder.
ECUs are becoming increasingly sophisticated, providing advanced diagnostic capabilities to detect problems within the system. The ECU microprocessor 19 monitors the fuel injection cycle to determine if the injector and injector controller are operating properly. As described above, when the fuel injector control MOSFET 16 is placed into a non-conducting state, the magnetic field in the fuel injector coil 14 collapses, causing a voltage spike that is greater than the supply voltage and that is applied to the fault detection circuit 20. The fault detection circuit 20 is operable to change the condition of the microprocessor feedback port 22 upon detection of a voltage spike. Logic within the ECU microprocessor is selectively operative to set an error flag in response to the changed condition on the microprocessor feedback port 22. Typically, an error flag is set if a voltage spike is detected when the injector is supposed to be off or if the voltage spike does not occur within an expected time period following the injector being turned off. Upon detection of a fault, a warning signal, such as the illumination of a warning light upon the vehicle dashboard, is generated to inform the vehicle operator that the engine is not operating properly. Additionally, a fault condition may place the ECU into a “limp home” mode and save an error code in memory for a later diagnostic.
The operation of the fault detection circuit 20 is illustrated in FIG. 2 where the voltage appearing at the drain terminal, VDRAIN, of MOSFET 16 is plotted as a function of time. Before time t1, the MOSFET 16 is in a non-conducting state and the voltage V+ appears at its drain terminal. At time t1, the MOSFET 16 is placed in a conducting state and the voltage at its drain terminal goes to zero. At a later time, t2, the MOSFET 16 is returned to a non-conducting state and the voltage V+ again appears at its drain terminal. Removal of the voltage from across the injector coil 14 causes the magnetic field within the coil to collapse at time t2, causing the voltage spike labeled 24 in FIG. 2. The time interval between t1 and t2 is labeled ΔT and represents the duration of the time that the fuel injector is open. The voltage spike 24 is detected by the fault detection circuit 20 which is operative to change the condition at the ECU microprocessor feedback port 22. As described above, logic within the ECU microprocessor 19 then determines whether the ECU 10 and/or the fuel injector are operating properly. While only one fuel injector coil 14 is shown in FIG. 1, it will be appreciated that a similar circuit is provided for each of the vehicle engine cylinders.
When modifications are made to a vehicle engine, such as replacing the exhaust system, the stock ECU 10 no longer provides the correct fuel amount across the engine's operating range. A resulting lean fuel/air mixture may be corrected by holding the fuel injector open for a longer period of time, i.e., by increasing the length of the voltage pulse applied to the fuel injector coil 14. Similarly, a resulting rich fuel/air mixture may be corrected by holding the fuel injector open for a shorter period of time, i.e., by decreasing the length of the voltage pulse applied to the fuel injector coil 14. However, as described above, the injector coil 14 is monitored by the ECU 10 and the ECU will erroneously conclude that the ECU injector closed either early or late and will generate an engine error code, indicating a fault in the injection circuit. Additionally, engine tuners may desire to change the timing of when the injector voltage pulse occurs relative to when the cylinder intake valve opens. Such changes may also trigger false error codes. Accordingly it would be desirable to provide a circuit that would allow varying the fuel injector pulse length and/or timing without triggering ECU error messages.